The term "fibrinolytic enzyme" means any enzyme that is capable of cleaving fibrin. Enzymes that are capable of cleaving fibrin include, but are not limited to, streptokinase, urokinase, tissue plasminogen activator (t-PA) produced from cell cultures, tissue plasminogen activator produced by recombinant DNA technology (rt-PA) and tissue plasminogen activator produced by prourokinase (k-PA). The terms "isotonic solution" or "isoosmotic solution" are defined as solutions having the same or similar osmotic pressure as blood. The terms clot, fibrin clot and thrombus are used interchangeably.
Each year about 550,000 Americans die from heart attacks. Even more--close to 700,000--have heart attacks and live. While a heart attack victim may survive, part of his or her heart will almost certainly die. The death of heart muscle, called myocardial infarction, is due to coronary artery thrombosis in 70-90% of the cases. When a thrombosis, or blood clot, occludes one of the arteries of the heart, it compromises the flow of blood to the surrounding muscle. This deprives the muscle of oxygen and other nutrients. In the past, nothing could be done to reverse this process. The high technology devices in intensive care units mostly supported patients so they could live while a portion of their heart died.
Similar situations occur in many other tissues when the blood supply to the tissue is affected by a thrombus or embolus. Stroke, deep vein thrombosis and pulmonary embolus are examples.
Another area where fibrinogen/fibrin plays a role is tumors. There is now strong evidence that fibrinogen-related proteins are localized in solid tumors. The anatomical distribution of fibrin in tumors varies depending on the tumor type. In carcinomas, fibrin is deposited in the tumor stroma and around tumor nests and may be particularly abundant toward the tumor periphery and at the tumor host interface. By contrast, fibrin is often less prominent in older, more central tumor stroma characterized by sclerotic collagen deposits. Fibrin may also be found between individual carcinoma cells. In some, but not all such cases, interepithelial fibrin deposits are related to zones of tumor necrosis; however, zones of tumor necrosis are not necessarily sites of fibrin deposition. Fibrin deposition in sarcomas has been less carefully studied than that in carcinomas. In lymphomas, fibrin deposits may be observed between individual malignant tumor cells as well as between adjacent, apparently reactive benign lymphoid elements. Fibrin has been reported to appear in zones of tumor sclerosis, as in Hodgkin's disease. Research has indicated that the pattern and extent of fibrin deposition are characteristic for a given tumor. (See Hemostasis and Thrombosis, Basic Principles and Clinical Practice, "Abnormalities of Hemostasis in Malignancy", pp. 1145-1157, ed. by R. W. Colman, et al., J. B. Lippincott Company, 1987).
The lack of a uniform vascular supply to tumors can impede diagnostic and therapeutic procedures. For example, hypoxic tumors are less susceptible to many drugs and to radiation. Conventional drugs and new drugs, such as monoclonal antibody conjugates, are not effective unless they are delivered to tumor cells. Fibrin deposits that surround some types of tumors inhibit delivery of the drugs to the tumor. The blood supply of tumors is further compromised by other factors as well. Blood vessels in tumors are frequently small and tortuous. The hydrodynamic resistance of such channels further impedes the flow of blood to tumors.
A similar situation occurs for different reasons during crisis of sickle cell anemia. Sickled red blood cells partially occlude small vessels producing local hypoxia and acidosis. This induces additional red blood cells to become sickled. The result is a vicious circle known as "crisis". Therapy involves increasing flow and oxygenation in affected areas. Another therapy involves the combination of calcium channel blockers. The formation of fibrin frequently complicates sickle cell crisis.
It has been found that certain enzymes are able act on fibrin deposits to open clogged arteries. The enzymes which have been used successfully include streptokinase, urokinase, prourokinase, tissue plasminogen activator produced from cell cultures and tissue plasminogen activator produced by recombinant DNA technology. These enzymes are most successful if administered shortly after the occlusion of the blood vessels before the heart tissue has sustained irreversible damage. In one study of 11,806 patients treated with intravenous or intracoronary artery streptokinase, an 18% improvement of survival was demonstrated. If the treatment was begun within one hour after the initial pain onset of the heart attack, the in-hospital mortality was reduced by 47%. (See The Lancet, Vol. 8478, p. 397-401, Feb. 22, 1986). It was demonstrated that early lysis of the thrombus resulted in salvage of a portion of heart tissue which would otherwise have died. In studies using angiography to assess the patency of blood vessels, it was found that tissue plasminogen activator could completely open the vessels of 61% of the 129 patients versus 29% of controls who were not treated with the enzyme. (See Verstraete, et al., The Lancet, Vol. 8462, p. 965-969), Nov. 2, 1985). Tissue plasminogen activator requires the addition of approximately 100 .mu.l of Tween 80 per liter of solution to promote dispersion of the enzyme. (See Korninger, et al., Thrombos, Haemostas, (Stuttgart) Vol. 46(2), p. 561-565 (1981)).
The enzymes used to lyse thrombi in vessels do so by activating fibrinolysis. Fibrin is the protein produced by polymerization of fibrinogen. It forms a gel which holds the thrombus together. The fibrin molecules which form clots gradually become cross-linked to make a more stable clot. All four enzymes, prourokinase, urokinase, streptokinase and tissue plasminogen activator, have similar effects on fibrin; however, they have different toxicities. If the fibrinolysis mechanisms are activated in the vicinity of a clot, the clot is lysed. If, however, they are activated systemically throughout the circulation, the body's capacity to stop bleeding or hemorrhage is markedly reduced. Streptokinase and urokinase tend to activate systemic fibrinolysis. Consequently, they have been most effective when injected directly into the affected blood vessel. Tissue plasminogen activator, in contrast, becomes effective only when it is actually attached to fibrin. This means its activity is largely localized to the immediate area of a clot and does not produce systemic fibrinolysis. If high doses are used in an effort to increase the rate of clot lysis or to lyse refractory clots, then the amount of systemic fibrinolysis and risk of hemorrhage become significant. It can be injected intravenously into the general circulation. It circulates harmlessly until it contacts the fibrin in a blood clot where it becomes activated and lyses the clot. Tissue plasminogen activator is able to lyse a clot which is extensively cross-linked. This means it is possible to lyse clots which have been present for many hours. Tissue plasminogen activator also produces less risk of hemorrhage than the other enzymes. Even more effective enzyme based thrombolytic drugs are being developed.
Remarkable as the new enzyme therapies are, they are subject to serious complications and are not effective in all patients. Clots in the anterior descending branch of the left coronary artery are much more readily lysed than those in other arteries. If the enzyme is not delivered by the blood stream directly to the thrombus, it has no effect. For various reasons, more blood passes by or trickles around thrombi in the left anterior descending coronary artery than in the other major arteries. In addition, the presence of collateral circulation which forms in response to compromised blood flow in the major arteries adversely affects the rate of reopening or recanalization of the thrombosed major arteries. It is thought the presence of many collateral vessels which allows blood to bypass the clot reduces the pressure gradient across the clot. This in turn reduces the blood flow through the tiny openings which may persist in the clot, impedes the delivery of enzymes to the clot, and prevents it from being lysed.
Once the clot is lysed, the factors which led to the formation of the thrombus persist. This produces a high incidence of re-thrombosis and further infarction in the hours and days following lysis of the clot. Rethrombosis has been reported in between 3% and 30% of cases in which the initial treatment successfully lysed the clot. Anticoagulants are currently used to prevent the formation of new thrombi, but they tend to induce hemorrhage. There is a delicate balance between the amount of anticoagulation necessary to prevent re-thrombosis of the vessels and that which will produce serious hemorrhage.
Finally, dissolving the clot after irreversible damage has taken place has little effect. The irreversible damage could be either to the heart muscle or vascular bed of the tissue supplied by the blood vessel. A major problem in widespread implementation of this new enzyme therapy is to find ways of identifying and treating the patients earlier in their disease and to find ways to make the treatment effective for a longer period of time after the initiation of thrombosis.
Animal studies have provided a better understanding of the events which control blood flow and tissue death following coronary artery thrombosis. The heart has several blood vessels, so much of the muscle receives blood from more than one vessel. For this and other reasons, the tissue changes following a coronary thrombosis are divided into distinct zones. The central zone of tissue becomes almost completely necrotic. This is surrounded by an area of severe ischemia. Outside this is an area of lesser ischemia called the marginal zone. Finally, there is a jeopardized zone which surrounds the entire area. In studies with baboons, the central necrotic area was not affected by recanalization of the vessel after several hours. However, muscle in the other zones which had undergone less severe damage during the ischemic period could be salvaged. A surprising finding was that lysing of the thrombus to produce a perfect arteriograph was insufficient to restore normal flow in the majority of animals. (See Flameng, et al, J. Clin. Invest., Vol. 75, p. 84-90, 1985).
Some further impediment to flow had developed in the area supplied by the vessel during the time that it was occluded. In further studies, it was demonstrated that immediately after removing the obstruction to the vessel, the flow through the damaged tissue began at a high rate. However, within a short time the blood flow through the ischemic zone decreased and the tissue died. Consequently, the regional blood flow immediately after reperfusion is a poor predictor of the salvage of myocardial tissue. If the blood flow through the damage tissue remained near the normal levels, the success of tissue salvage was much greater. Hemorrhage occurred almost exclusively in the severely ischemic zone reflecting damage to the small blood vessels. The hemorrhage, however, remained limited to the severely ischemic tissue and did not cause extension of the infarction or other serious complication. Therapies which could preserve the blood flow through the small blood vessels distal to the major area of thrombus after reperfusion could be expected to markedly increase the salvage of myocardial tissue.
The damage to heart muscle cells which occurs after lysing the thrombus is due to other factors as well as ischemia. Contact of fresh blood with damaged or dead cells induces the influx of neutrophils, or pus cells, which kill heart cells which would otherwise have recovered. Much of the damage caused by neutrophils has been attributed to superoxide ions. The superoxide anion can damage tissue in several ways. The interaction of the superoxide anion with hydrogen peroxide leads to the production of hydroxyl radicals which are potentially toxic and react rapidly with most organic molecules. Mannitol is a selective scavenger of hydroxyl radicals. The enzyme, superoxide dismutase, catalyzes the decomposition of the superoxide anion. Enzymes such as superoxide dismutase, free radical scavengers or agents which prevent the influx on neutrophils are able to increase the salvage of heart muscle cells.
Low concentrations of copolymers have little effect on plasma proteins. Higher concentrations, above the critical micelle concentration, activate complement via the alternate pathway. This provides further benefit for treating heart attacks because the systemic activation of complement causes the neutrophils to become unresponsive to complement chemotaxis. This prevents their migration into the heart tissue.
Continuing therapy is needed even after restoration of blood flow and salvage of damaged tissue. The arteriosclerosis that caused the original heart attack remains. American and European researchers have found that arteriosclerosis still narrows the arteries in 70-80% of patients whose clots were lysed by thrombolytic therapy. Many physicians believe this obstruction must be opened for long term benefits. Balloon angioplasty is a procedure whereby a catheter with a small balloon is inserted into the narrowed artery. The balloon is inflated, compresses the atherosclerotic plaque against the vessel wall and dilates the artery. The effectiveness of this procedure is limited by the effects of ischemia produced by the balloon, by embolization of atheromatous material which lodges in distal vessels and by an increased tendency for immediate or delayed thrombosis in the area damaged by the balloon. The balloon tears the tissue exposing underlying collagen and lipid substances which induce formation of thrombi. The thrombus may occlude the vessel immediately or set up a sequence of events which leads to occlusion many days or weeks later. What is needed is a means of rendering the surface of the dilated vessel less thrombogenic, improving the blood flow through the distal tissue and breaking the embolized material into smaller pieces which are less likely to produce embolic damage.
Finally, lipid material on the atherosclerotic wall contributes to the bulk of the plaque which narrows the lumen of the artery and produces a highly thrombogenic surface. What is needed is a method of extracting or covering lipids from atherosclerotic plaques which leaves their surfaces less thrombogenic and reduces their bulk.
Use of copolymers prepared by the condensation of ethylene oxide and propylene oxide to treat an embolus or a thrombus has been described (See U.S. Pat. No. 3,641,240). However, the effect was limited to recently formed, small (preferably microscopic) thrombi and emboli which are composed primarily of platelets. The use of the ethylene oxide and propylene oxide copolymer has little or no effect on a clot in a patient who has suffered a severe coronary infarction. The clots that form in these patients are large and stable clots. Stable clots are clots in which the fibrin that has formed from fibrinogen has undergone cross linking. Fibrin which has undergone crosslinking is not effected by presence of the ethylene oxide-propylene oxide copolymers. The copolymers only affect new clots in which the newly formed fibrin has not crosslinked.
Thus, a composition is needed that is capable of lysing a clot and, at the same time, will prevent a second clot from reforming after the initial clot has been cleared. Ideally, such a composition would also reduce as much as possible any damage that is caused by blockage of blood supply to the tissue. Such a composition would thereby protect the patient from any damage caused by the reformation of a clot. In addition, such a composition would be useful in removing clots from solid tumors, increasing flow through tortuous channels and thereby allow delivery of therapeutic drugs to the tumor.